TWI482346B - Method of manufacturing cathode material for lithium secondary battery - Google Patents

Description

Method for producing positive electrode active material for lithium secondary battery

The present invention relates to a method for producing a positive electrode active material for a lithium secondary battery.

The present application claims priority based on Japanese Patent Application No. 2011-102090, filed on Jan.

Lithosilicate-type Lithium Metal Phosphate LiMPO ₄ (M is a transition metal) is cheaper because it can produce LiCoO ₂ which has a relatively large electrical capacity and is widely used as a positive electrode active material of a lithium secondary battery. In the case of a lithium battery, in particular, a positive electrode active material of a large-sized lithium secondary battery such as an automobile, it is expected (refer to Patent Documents 1 and 2).

As a synthesis method of LiMPO ₄ , various synthesis methods such as a solid phase method, a hydrothermal method, and a coprecipitation method have been proposed, but in order to remove impurities contained in the obtained LiMPO ₄ to improve purity, usually The lithium metal phosphate (LiMPO ₄ ) is washed after the synthesis (refer to Patent Documents 3 and 4).

Patent Documents 3 and 4 disclose that a positive electrode active material for a lithium secondary battery containing lithium metal phosphate is washed by using a pH buffer to remove impurities and improve purity. Revealing the particular, when the lithium metal phosphate is LiFePO _4, LiFePO Fe ₄ without dissolved, but may contain such reducing _{FeSO 4, FeO, Fe 3 (} PO 4) ₂ as the two valence iron impurities, and, Impurities such as Li ₃ PO ₄ or Li ₂ CO ₃ contained in LiFePO ₄ can also be reduced.

[Previous Technical Literature] [Patent Document]

[Patent Document 1] Japanese Patent Publication No. 9-171827

[Patent Document 2] Japanese Patent Publication No. 9-134725

[Patent Document 3] Japanese Patent Laid-Open Publication No. 2009-32656

[Patent Document 4] Japanese Patent Laid-Open Publication No. 2009-259807

In the above-mentioned Patent Documents 3 and 4, the cleaning liquid for suppressing the dissolution of Fe in the lithium iron phosphate LiFePO ₄ is disclosed, but the cleaning liquid for suppressing the dissolution of Li in LiFePO ₄ is not Specifically disclosed or taught.

On the other hand, it is known that when water is used to wash the synthesized lithium metal phosphate LiMPO ₄ , the discharge capacity of the lithium secondary battery used as the positive electrode active material when the cleaning time becomes longer will be reduce.

The results in Table 1 are for lithium iron phosphate LiFePO ₄ .

The discharge capacity shown in Table 1 was repeated at a constant temperature of 25 ° C at a constant current of 0.1 C to 3.9 V, and then a constant current discharge to a charge and discharge cycle of 2.3 V at a current of 0.1 C. It is the result of the measurement of the discharge capacity of the second time.

As can be seen from Table 1, although the discharge capacity was increased when the water washing time was extended to 1 minute and 2 minutes, the discharge capacity was lowered after the next 3 minutes. The reason why the discharge capacity is lowered as the washing time becomes longer is that the lithium iron phosphate LiFePO ₄ powder color changes from light green to blue when the water washing time is prolonged, so it is presumed that lithium ions will be washed during washing. The surface of the lithium iron phosphate LiFePO ₄ is eluted, and a heterogeneous layer is formed on the surface, and the heterogeneous layer hinders the diffusion of lithium ions.

The present invention has been made in view of the above-mentioned problems, and is directed to a method for producing a positive electrode active material for a lithium secondary battery comprising an olivine-type lithium metal phosphate, which is obtained by synthesizing an olivine-type lithium metal phosphate. At the time of washing, the lithium ion is prevented from being eluted by the lithium metal phosphate, and a lithium secondary battery having an increased charge/discharge capacity and a discharge rate can be formed.

In order to achieve the above object, the present invention provides the following means.

[1] A method for producing a positive electrode active material for a lithium secondary battery, which comprises synthesizing a lithium metal represented by a composition formula of LiMPO ₄ (the element M is a transition metal of any one or more of Fe, Mn, Co or Ni) After the phosphate, the lithium metal phosphate is washed with a lithium ion-containing cleaning solution.

[2] The method for producing a positive electrode active material for a lithium secondary battery according to the above [1], wherein the solute of the cleaning liquid contains LiClO ₄ , Li ₂ CO ₃ , LiOH, LiPF ₆ , Li ₃ PO ₄ , LiH ₂ PO _4. At least one of CH ₃ CO ₂ Li.

[3] The method for producing a positive electrode active material for a lithium secondary battery according to any one of the above [1], wherein the solvent of the cleaning liquid is a liquid containing water.

[4] The method for producing a positive electrode active material for a lithium secondary battery according to the above [3], wherein the aqueous liquid is water.

[5] The method for producing a positive electrode active material for a lithium secondary battery according to any one of the above [1] to [4] wherein the pH of the lithium ion-containing cleaning liquid is 5 or more and 9 or less.

The method for producing a positive electrode active material for a lithium secondary battery according to any one of the above aspects, wherein the cleaning system sets the temperature of the lithium ion-containing cleaning liquid to 15 ° C or higher. get on.

[7] The method for producing a positive electrode active material for a lithium secondary battery according to any one of the above [1], wherein the washing time is within 1 hour.

[8] The method for producing a positive electrode active material for a lithium secondary battery according to any one of the above [1], wherein the washing comprises a step of stirring the lithium ion-containing cleaning liquid.

[9] The method for producing a positive electrode active material for a lithium secondary battery according to any one of the above [1] to [8] wherein the lithium metal phosphate after the synthesis contains either or both of Nb or V.

[10] The method for producing a positive electrode active material for a lithium secondary battery according to any one of the above [1] to [9] wherein the particle size of the lithium metal phosphate after the synthesis is 20 to 200 nm.

[11] The method for producing a positive electrode active material for a lithium secondary battery according to any one of the above [1] to [10] wherein, in the region from the surface of the synthesized lithium metal phosphate to a depth of 2 nm, The ratio of the number of Li atoms of the number of M atoms is 0.7 or more and less than 1.1.

[12] The method for producing a positive electrode active material for a lithium secondary battery according to any one of the above [1], wherein, after the washing, the lithium metal phosphate and a liquid in which a substance containing a carbon atom is dissolved mixing.

[13] The method for producing a positive electrode active material for a lithium secondary battery according to the above [12], wherein after the mixing, the solvent in which the liquid containing the carbon atom-containing substance is dissolved is removed.

[14] The method for producing a positive electrode active material for a lithium secondary battery according to the above [13], wherein, after removing the solvent of the liquid in which the substance containing a carbon atom is dissolved, a mixture of a lithium metal phosphate and a substance containing a carbon atom is The atmosphere is heated at a temperature of 400 ° C or higher and 900 ° C or lower for 1 hour or longer and 20 hours or shorter in an atmosphere having an oxygen concentration of 1% or less.

[15] The method for producing a positive electrode active material for a lithium secondary battery according to any one of the above [1], wherein, after the washing, the carbon-containing layer is formed on at least a surface of the lithium metal phosphate. a part.

[16] The method for producing a positive electrode active material for a lithium secondary battery according to any one of the above [1] to [15] wherein the transition metal (M) of the LiMPO ₄ is either Fe or Mn.

[17] The method for producing a positive electrode active material for a lithium secondary battery according to any one of the above [1], wherein the transition metal (M) of the LiMPO ₄ contains both Fe and Mn.

[18] The method for producing a positive electrode active material for a lithium secondary battery according to any one of the above [1] to [17] wherein the synthesis is carried out by hydrothermal synthesis.

[19] The method for producing a positive electrode active material for a lithium secondary battery according to the above [18], wherein the hydrothermal synthesis is a liquid containing water, a source of lithium (Li), a source of one or more transition metals (M), The phosphoric acid (PO ₄ ) source is used as a raw material.

[20] The method for producing a positive electrode active material for a lithium secondary battery according to the above [18], wherein the hydrothermal synthesis uses a liquid containing water, Li ₃ PO _{4 ,} and one or more kinds of transition metal (M) sources as raw materials. Come on.

[21] The method for producing a positive electrode active material for a lithium secondary battery according to any one of the above [19], wherein the transition metal (M) source is FeSO ₄ and/or MnSO ₄ .

[22] The method for producing a positive electrode active material for a lithium secondary battery according to any one of the above [19] or [20], wherein both of FeSO ₄ and MnSO ₄ are used as the source of the transition metal (M).

[23] The method for producing a positive electrode active material for a lithium secondary battery according to any one of the above [19], wherein the lithium (Li) source is LiOH.

[24] The method for producing a positive electrode active material for a lithium secondary battery according to any one of the above [19], wherein the source of the phosphoric acid (PO ₄ ) is H ₃ PO ₄ .

According to the present invention, there is provided a method for producing a positive electrode active material for a lithium secondary battery comprising an olivine-type lithium metal phosphate, which is capable of suppressing lithium ions when washed by synthesizing an olivine-type lithium metal phosphate. The lithium metal phosphate is eluted to form a lithium secondary battery having an increased charge and discharge capacity and a discharge rate.

[Best form of implementing the invention]

Hereinafter, a method for producing a positive electrode active material for a lithium secondary battery according to an embodiment of the present invention will be described.

(Manufacturing method of positive electrode active material for lithium secondary battery)

A method for producing a positive electrode active material for a lithium secondary battery according to a preferred embodiment of the present invention is characterized in that after the lithium metal phosphate represented by the composition formula LiMPO _{4 is} synthesized, the lithium ion-containing cleaning liquid is used to wash the foregoing Lithium metal phosphate. Here, the element M of LiMPO ₄ is any one or two or more kinds of transition metals of Fe, Mn, Co or Ni.

The method for synthesizing the lithium metal phosphate is not particularly limited, and a hydrothermal synthesis method, a solid phase synthesis method, or the like can be used.

The solute of the lithium ion-containing cleaning liquid is not particularly limited as long as it is dissolved to form lithium ions, and preferably contains LiClO ₄ , Li ₂ CO ₃ , LiOH, LiPF ₆ , Li ₃ PO ₄ , LiH ₂ PO ₄ , At least one of CH ₃ CO ₂ Li.

The solute of the lithium ion-containing cleaning liquid may also contain those which do not generate lithium ions.

The lithium ion concentration of the lithium ion-containing cleaning liquid is preferably 0.01 mol/L or more, more preferably 1 mol/L or more. If it is less than 0.01 mol/L, the effect of suppressing the elution of lithium ions from the lithium metal phosphate LiMPO ₄ is too small, so it is difficult to attempt to increase the charge and discharge capacity and the discharge rate; if it is 1 mol/L or more, the lithium ion dissolution is suppressed. The effect is large, so that the charge and discharge capacity and the discharge rate can be sufficiently increased.

The solvent of the washing liquid is preferably an aqueous liquid, more preferably water. The purpose of washing with a lithium ion-containing cleaning liquid is to remove the synthetic raw material contained in the synthesized lithium metal phosphate as ion elution. For example, when the lithium metal phosphate LiFePO ₄ is synthesized using FeSO ₄ as the transition metal (M) source, the raw material contained in the lithium metal phosphate LiFePO ₄ after the synthesis is removed as a sulfate ion or the like. .

The pH of the lithium ion-containing cleaning liquid is preferably 5 or more and 9 or less. When the pH is 5 or less or 9 or more, the metal M is easily eluted from LiMPO ₄ .

The pH adjustment can be carried out by adding sulfuric acid or ammonia water to the cleaning solution. The pH can be measured by a pH meter using the glass electrode method as a principle.

The washing with a lithium ion-containing cleaning liquid is preferably carried out by setting the temperature to 15 ° C or higher. If it is less than 15 ° C, the efficiency of removing impurities mixed in LiMPO ₄ is lowered.

The washing time is preferably within one hour. If it is longer than 1 hour, it is dissolved in the oxygen in the cleaning liquid, and LiMPO ₄ is oxidized.

In the washing, it is preferred to include a step of stirring a lithium ion-containing cleaning liquid. By stirring, impurities existing in the mixture with LiMPO ₄ can be more dissolved in the cleaning liquid, and the residual of impurities can be suppressed.

Preferably, the lithium metal phosphate after the synthesis contains either or both of Nb or V. When the Nb or V is repeatedly charged and discharged, the capacity reduction can be suppressed.

A lithium metal phosphate containing Nb or V can be produced by adding a substance containing Nb or V to a synthetic raw material of lithium metal phosphate. As the substance containing Nb, for example, phenolphthalein (Nb(OC ₆ H ₅ ) ₅ or cerium chloride (NbCl ₅ )) is exemplified, and as the substance containing V, for example, ammonium vanadate (NH ₄ VO ₃ ) is exemplified.

The particle diameter of the synthesized lithium metal phosphate is preferably from 20 to 200 nm. If it is smaller than 20 nm, the crystallinity is lowered and the capacity is lowered. If it is larger than 200 nm, the speed of charge and discharge is lowered.

The particle size of the lithium metal phosphate can be adjusted to 20 to 200 nm by adjusting the pH of the synthesized liquid. However, depending on the concentration of the lithium (Li) source, the transition metal (M) source, the phosphoric acid (PO ₄ ) source, the type of the transition metal (M) source, the synthesis temperature, the synthesis time, and the stirring strength, the pH should be adjusted. The range will change. As a method of adjusting the pH, for example, sulfuric acid or ammonia water is added to the raw material.

The ratio of the number of Li atoms to the number of M atoms in the region from the surface of the synthesized lithium metal phosphate to the depth of 2 nm is preferably 0.7 or more and less than 1.1.

The ratio of the number of Li atoms to the number of M atoms can be measured, for example, by TEM-EELS (Electron Energy Loss Spectrum).

In the lithium metal phosphate synthesized by the usual method, The ratio of the number of Li atoms to the number of M atoms is close to 1. However, when the synthesized lithium metal phosphate is washed as in the prior art, when water is used, Li atoms are eluted from the vicinity of the surface of the lithium metal phosphate, and the number of Li atoms with respect to the metal atom M is reduced. Therefore, in the region from the surface of the lithium metal phosphate to a depth of 5 nm, the ratio of the number of Li atoms to the number of M atoms becomes smaller than 1. In contrast, when the cleaning of the lithium ion-containing cleaning liquid of the present invention is appropriately performed, the elution of Li atoms in the vicinity of the surface can be suppressed, and as a result, the number of Li atoms relative to the number of M atoms is considered to be The range of variation (the range from the surface to the depth direction) is thus smaller. However, the difference in composition ratio will be greater than 1 (1~1.1), which is believed to be caused by factors other than washing. Further, the ratio of the number of Li atoms to the number of M atoms after the synthesis of the lithium metal phosphate and before the washing is usually in the range of 0.9 to 1.1.

After washing, it is preferred to mix the lithium metal phosphate with a liquid in which a substance containing a carbon atom is dissolved.

Since the carbon-containing layer can be formed on at least a portion of the surface of the lithium metal phosphate, the conductivity can be improved.

The liquid which dissolves the substance containing a carbon atom is not particularly limited as long as it has a low reactivity with LiMPO ₄ , and for example, water, ethanol or acetone can be used.

After the mixing, it is more preferable to remove the solvent of the liquid in which the substance containing a carbon atom is dissolved.

Since the carbon-containing layer having less impurities can be formed on at least a portion of the surface of the lithium metal phosphate, the conductivity can be further improved.

After removing the solvent of the liquid in which the substance containing a carbon atom is dissolved, it is preferred to use a mixture of a lithium metal phosphate and a substance containing a carbon atom in an atmosphere having an oxygen concentration of 1% or less, and to use 400 ° C or more and 900 ° C or less. The temperature is heated for 1 hour or more and 20 hours or less.

If the oxygen concentration is higher than 1%, LiMPO ₄ will oxidize. Further, when it is lower than 400 ° C, the carbon atom content of the carbon-containing layer formed on the surface of the lithium metal phosphate is lowered, and the electrical conductivity is lowered; if it is higher than 900 ° C, LiMPO ₄ is The crystal grains will grow and the speed of charge and discharge will decrease. If it is shorter than 1 hour, the carbon atom content of the carbon-containing layer formed on the surface of the lithium metal phosphate will decrease, and the electrical conductivity will decrease; if it is longer than 20 hours, the furnace will be heated. Energy will become wasteful.

Preferably, the carbon-containing layer is formed on at least a portion of the surface of the lithium metal phosphate after washing.

Because it can improve conductivity.

As the transition metal (M) constituting the lithium metal phosphate LiMPO ₄ , any one or two or more of Fe, Mn, Co or Ni may be used, but it is particularly preferably either Fe or Mn. That is, as the lithium metal phosphate, any of LiFePO ₄ or LiMnPO ₄ or LiFeM'PO ₄ (M' is another transition metal other than Fe) or LiMnM'PO ₄ (M' is not Mn is preferable. Any of the other transition metals). Further, the transition metal (M) constituting the lithium metal phosphate LiMPO ₄ preferably contains both Fe and Mn. That is, as the lithium metal phosphate, LiFeMnPO ₄ or LiFeMnM'PO ₄ (M' is any other transition metal which is not any of Fe and Mn) is preferable.

Since both LiFePO ₄ and LiMnPO _{4 have} a high theoretical capacity (170 mAh/g for LiFePO _{4 and} 171 mAh/g for LiMnPO ₄ ), either LiFePO ₄ or LiMnPO ₄ or LiFeM'PO ₄ (M' is used. Any other transition metal that is not Fe) or LiMnM'PO ₄ (M' is another transition metal that is not Mn), or LiFeMnPO ₄ or LiFeMnM'PO ₄ (M' is not Fe and Mn One of the other transition metals) can increase the battery capacity per unit mass.

The synthesis of the lithium metal phosphate LiMPO ₄ before washing is preferably carried out by hydrothermal synthesis. Since the hydrothermal synthesis method can obtain LiMPO ₄ having a small particle diameter at a relatively low temperature and for a short period of time.

Hydrothermal synthesis of lithium metal phosphate LiMPO ₄ before washing, using aqueous liquid, lithium (Li) source, one or more transition metal (M) sources, and phosphoric acid (PO ₄ ) source as raw materials get on.

Further, the hydrothermal synthesis of the lithium metal phosphate LiMPO ₄ before washing can be carried out by using a liquid containing water, Li ₃ PO ₄ and one or more kinds of transition metal (M) sources as raw materials.

In the above hydrothermal synthesis, either FeSO ₄ or MnSO ₄ may be used as the transition metal (M) source. Thereby, LiFePO ₄ or LiMnPO ₄ as lithium metal phosphate, or LiFeM'PO ₄ (M' is another transition metal other than Fe) or LiMnM'PO ₄ (M' is another transition metal other than Mn can be synthesized. ).

In the above hydrothermal synthesis, both FeSO ₄ and MnSO ₄ may be used as the transition metal (M) source. Thereby, LiFeMnPO ₄ as a lithium metal phosphate or LiFeMnM'PO ₄ (M' is another transition metal which is not any of Fe and Mn) can be synthesized.

Further, in the above hydrothermal synthesis, LiOH can be used as a lithium (Li) source.

Further, in the above hydrothermal synthesis, H ₃ PO ₄ can be used as a phosphoric acid (PO ₄ ) source.

(lithium battery)

A lithium secondary battery according to an embodiment of the present invention includes a positive electrode, a negative electrode, and a nonaqueous electrolyte. In the lithium secondary battery, as the positive electrode active material contained in the positive electrode, those produced by the above method can be used. By providing such a positive active material, the capacity and discharge rate of the lithium secondary battery can be increased. Hereinafter, the positive electrode, the negative electrode, and the nonaqueous electrolyte constituting the lithium secondary battery will be described in order.

(positive electrode)

In the lithium secondary battery of the present embodiment, as the positive electrode, a sheet made of a positive electrode mixture (containing a positive electrode active material and a conductive auxiliary material and a binder) and a positive electrode current collector bonded to the positive electrode mixture can be used. Electrode. Further, as the positive electrode, a pellet-type or sheet-like positive electrode obtained by molding the above-mentioned positive electrode mixture into a disk shape may be used.

The lithium metal phosphate produced by the above method can be used as the positive electrode active material. However, the lithium metal phosphate may be used by mixing a conventional positive electrode active material.

As the binder, for example, polyethylene, polypropylene, ethylene propylene copolymer, ethylene propylene triene copolymer, butadiene rubber, styrene butadiene rubber, butyl rubber, polytetrafluoroethylene, poly(methyl) can be exemplified. Acrylate, polyvinylidene fluoride, polyethylene oxide, polypropylene oxide, polyepichlorohydrin, polyphosphazene, polyacrylonitrile, and the like.

Further, examples of the conductive auxiliary material include conductive metal powder such as silver powder; conductive carbon powder such as furnace black, KETLENBLACK, and acetylene black; carbon nanotubes, nano carbon fibers, and fumed carbon fibers. As the conductive auxiliary agent, a gas phase carbon fiber is preferred. The carbon fiber of the vapor-phase method preferably has a fiber diameter of 5 nm or more and 0.2 μm or less. The fiber length/fiber diameter ratio is preferably from 5 to 1,000. The content of the vapor-phase carbon fiber is preferably from 0.1 to 10% by mass based on the dry mass of the positive electrode mixture.

Further, examples of the positive electrode current collector include a foil of a conductive metal, a mesh of a conductive metal, and a stamped metal of a conductive metal. As the conductive metal, aluminum or an aluminum alloy is preferable.

Further, in the positive electrode mixture, an ion conductive compound, a tackifier, a dispersant, a lubricant, or the like may be contained as needed. Examples of the ion conductive compound include polysaccharides such as chitin and chitosan, and crosslinked products of the polysaccharides. Examples of the tackifier include carboxymethylcellulose, polyvinyl alcohol, and the like.

For example, a paste-form positive electrode mixture material is applied to a positive electrode current collector, dried, and subjected to pressure molding, or a powdery particle-shaped positive electrode mixture material is subjected to pressure molding on a positive electrode current collector. The positive electrode can be obtained. The thickness of the positive electrode is usually 0.04 mm or more and 0.15 mm or less. The positive electrode of any electrode density can be obtained by adjusting the pressure applied during molding. The pressure applied during molding is preferably from about ¹ t/cm ^{2 to} about 3 t/cm ² .

(negative electrode)

As the negative electrode, a sheet-shaped electrode formed of a negative electrode mixture (containing a negative electrode active material, a binder, and a conductive auxiliary material added as needed) and a negative electrode current collector bonded to the negative electrode mixture can be used. Further, as the negative electrode, a pellet-shaped or flake-shaped negative electrode obtained by molding the above-mentioned negative electrode mixture into a disk shape may be used.

As the negative electrode active material, a conventional negative electrode active material such as a black material such as artificial black lead or natural black lead or a metal such as Sn or Si or a semi-metal material can be used.

As the binder, the same as the binder used in the positive electrode can be used.

Further, the conductive auxiliary material may be added as needed, or may not be added. Conductive carbon powder such as furnace black, KETLENBLACK, acetylene black, or the like; a carbon nanotube, a carbon fiber, a vapor-phase carbon fiber, or the like can be used. As the conductive auxiliary agent, a gas phase carbon fiber is particularly preferred. The carbon fiber of the vapor-phase method preferably has a fiber diameter of 5 nm or more and 0.2 μm or less. The fiber length/fiber diameter ratio is preferably from 5 to 1,000. The content of the vapor-phase carbon fiber is preferably from 0.1 to 10% by mass based on the dry mass of the negative electrode mixture.

Further, examples of the negative electrode current collector include a foil of a conductive metal, a mesh of a conductive metal, and a stamped metal of a conductive metal. As the conductive metal, an alloy of copper or copper is preferable.

For example, a paste-form negative electrode mixture material is applied to a negative electrode current collector, dried, and subjected to pressure molding, or a powdery particle-shaped negative electrode mixture material is subjected to pressure molding on a negative electrode current collector. A negative electrode can be obtained. The thickness of the negative electrode is usually 0.04 mm or more and 0.15 mm or less. A negative electrode of any electrode density can be obtained by adjusting the pressure applied during molding. The pressure applied during molding is preferably from about ¹ t/cm ^{2 to} about 3 t/cm ² .

(non-aqueous electrolyte)

Next, examples of the nonaqueous electrolyte include a nonaqueous electrolyte in which a lithium salt is dissolved in an aprotic solvent.

The aprotic solvent is preferably selected from the group consisting of ethyl carbonate, diethyl carbonate, dimethyl carbonate, methyl ethyl carbonate, propyl carbonate, butyl carbonate, γ-butyrolactone, And at least one or a mixture of two or more of the groups formed by the carbonic acid extending vinyl ester.

Further, examples of the lithium salt include LiClO ₄ , LiPF ₆ , LiAsF ₆ , LiBF ₄ , LiSO ₃ CF ₃ , CH ₃ SO ₃ Li, CF ₃ SO ₃ Li, and the like.

Further, as the nonaqueous electrolyte, a so-called solid electrolyte or a gel electrolyte can also be used.

Examples of the solid electrolyte or the gel electrolyte include a polymer electrolyte such as a sulfonation styrene-olefin copolymer, a polymer electrolyte using polyethylene oxide and MgClO ₄ , and a structure having a propylene oxide (trimethylene oxide). Polymer electrolytes, etc. The nonaqueous solvent used in the polymer electrolyte is preferably selected from the group consisting of ethyl carbonate, diethyl carbonate, dimethyl carbonate, methyl ethyl carbonate, propyl carbonate, and butyl carbonate. At least one of the group consisting of γ-butyrolactone and vinyl carbonate.

Further, the lithium secondary battery of the present embodiment is not limited to the positive electrode, the negative electrode, and the nonaqueous electrolyte, and may be provided with other members or the like as needed. For example, a separator for isolating the positive electrode from the negative electrode may be provided. The separator is necessary when the nonaqueous electrolyte is a non-polymer electrolyte, and examples thereof include a nonwoven fabric, a woven fabric, a microporous film, and the like, or the like, and more specifically, a porous polypropylene film can be suitably used. A porous polyethylene film or the like.

The lithium secondary battery of the present embodiment can be used in various fields. For example, personal computers, tablets, notebook computers, mobile phones, wireless devices, electronic organizers, electronic dictionaries, PDA (Personal Digital Assistant), electronic instruments, electronic keys, electronic tags, power storage devices, power tools, Electrical appliances, digital cameras, digital video recorders, AV equipment, vacuum cleaners, etc.; electric vehicles, hybrid electric vehicles, electric motor vehicles, hybrid electric vehicles, electric bicycles, electric bicycles, railways, aircraft, ships, etc. Vehicles; solar power generation systems, wind power generation systems, tidal power generation systems, geothermal power generation systems, thermal power generation systems, vibration power generation systems, etc.

[Examples] (Example 1) (1) Hydrothermal synthesis

In a nitrogen atmosphere glove box with an oxygen concentration of 0.5% or less, 123 g of LiOH is used. H ₂ O (Deer grade manufactured by Kanto Chemical Co., Ltd.) was dissolved in 700 mL of water and stirred. Further, 113 g of a concentration of 85% H ₃ PO ₄ (a special grade 85.0% aqueous solution manufactured by Kanto Chemical Co., Ltd.) was slowly added while stirring. This is called liquid A. Next, in a glove box, 1.82 g of ascorbic acid was dissolved in 700 mL of water, and then 272 g of FeSO ₄ was dissolved. 7H ₂ O (Special grade produced by Kanto Chemical Co., Ltd.). This is called liquid B. Thereafter, the A liquid and the B liquid were mixed in a glove box, and after being stirred, they were placed in an autoclave and sealed. The autoclave was heated from room temperature to 200 ° C for 1 hour, and then heated at 200 ° C for 3 hours to synthesize LiFePO ₄ powder. It is then naturally cooled.

(2) Washing

Next, after the autoclave was cooled to room temperature, the produced LiFePO _{4 was} taken out, and a liquid (Li concentration: 1 mol/L) in which Li ₂ SO _{4 was} dissolved in a ratio of 5.488 g with respect to 100 g of ion-exchanged water was used, and it took 3 minutes. Filter and wash.

(3) Drying

Next, the washed powder was heated to 100 ° C in a vacuum and dried for 5 hours.

(4) Carbon coating

Next, 25 g of sucrose was dissolved in 100 g of water to prepare a sucrose solution, and 50 g of a sucrose solution was added to 100 g of the dried LiFePO ₄ powder, followed by stirring. The LiFePO ₄ powder to which the sucrose solution was added was heated at 100 ° C for 5 hours in a vacuum and dried. The dried LiFePO ₄ powder was heated from room temperature to 700 ° C in nitrogen for 90 minutes, and then heated for 5 hours, and then carbon was coated around the LiFePO ₄ particles by natural cooling. The carbon-coated LiFePO ₄ powder was pulverized using a pulverizer (A10 manufactured by IKA Corporation).

(5) Positive plate production

Next, ₅ g of acetylene black (manufactured by Electrochemical Industry Co., Ltd.) as a conductive auxiliary agent and PVDF (polyvinylidene fluoride) as a binder were added to 90 g of the pulverized LiFePO ₄ powder (Wu Yu Chemical Industry Co., Ltd.) 5 g of NMP (N-methyl-2-pyrrolidone) (manufactured by Kishida Chemical Co., Ltd.) as a solvent, and kneaded until it became uniform. The kneaded mixture was applied to an Al foil at a thickness of 30 μm, and dried at 90° C., and then pressed at a density of 2.2 g/cm ^{3 of the} positive electrode material to obtain a positive electrode plate.

(6) Negative plate production

Next, 5 g of acetylene black (manufactured by Electrochemical Industry Co., Ltd.) as a conductive auxiliary agent was added as a conductive auxiliary agent to 95 g of mesocarbon microbeads (manufactured by Osaka Gas Co., Ltd.) as a negative electrode material. 5 g of the PVDF (manufactured by Kureha Chemical Industry Co., Ltd.) and 50 g of NMP (manufactured by Kishida Chemical Co., Ltd.) as a solvent were kneaded until they became uniform. The kneaded mixture was applied to a Cu foil at a thickness of 35 μm, dried at 90 ° C, and then pressed at a density of 1.5 g/cm ^{3 of the} negative electrode material to obtain a negative electrode plate.

(7) Battery production

Next, a polypropylene microporous film (Celgard 2400 (manufactured by Hoechst Celanese)) and a negative electrode plate as separators were superposed on the positive electrode plate of the test, and an EC having a volume ratio of 2:3 as an electrolytic solution was used. A mixture of ethylene carbonate and EMC (ethyl methyl carbonate) was placed in a liquid in which 1.0 mol/L of LiPF ₆ was dissolved, and sealed in a coin battery.

(8) Charge and discharge test

At a temperature of 25 ° C, repeat the constant current constant voltage charging to 3.9 V with a current of 0.02 C, and then perform constant current discharge to a charge and discharge of 2.3 V with currents of 0.1 C, 0.2 C, 0.5 C, 1 C, and 2 C, respectively. The cycle was repeated twice, and the discharge capacity of the second cycle was measured. The discharge capacities of 0.1C, 0.2C, 0.5C, 1C, and 2C were 156.4 mAh/g, 155.6 mAh/g, 152.1 mAh/g, 147.3 mAh/g, and 139.5 mAh/g, respectively. The evaluation results are shown in Table 2.

(Example 2)

In the case of Example 2, a battery for evaluation was produced under the same conditions as in Example 1 except for the washing step.

The contents of the specific washing step and the results of the charge and discharge test are as follows.

(1) Washing

After the autoclave was cooled to room temperature, the produced LiFePO _{4 was} taken out, and a liquid (Li concentration: 0.1 mol/L) in which Li ₂ CO _{3 was} dissolved in a ratio of 0.370 g with respect to 100 g of ion-exchanged water was used, and it took 3 minutes. Filter and wash.

(2) Charge and discharge test

In the same manner as in the first embodiment, the constant current constant voltage was charged to 3.9 V at a temperature of 0.25 C at a temperature of 25 ° C, and then a constant current was performed at a current of 0.1 C, 0.2 C, 0.5 C, 1 C, and 2 C, respectively. The discharge was discharged to a charge and discharge cycle of 2.3 V twice, and the discharge capacity of the second time was measured. The discharge capacities of 0.1 C, 0.2 C, 0.5 C, 1 C, and 2 C were 152.6 mAh/g, 150.8 mAh/g, 146.6 mAh/g, 142.4 mAh/g, and 134.9 mAh/g, respectively. The evaluation results are shown in Table 2.

(Example 3)

In the case of Example 3, a battery for evaluation was produced under the same conditions as in Example 1 except for the washing step.

The contents of the specific washing step and the results of the charge and discharge test are as follows.

(1) Washing

After the autoclave was cooled to room temperature, the produced LiFePO _{4 was} taken out, and a liquid (Li concentration: 0.01 mol/L) in which Li ₃ PO _{4 was} dissolved in a ratio of 0.0386 g with respect to 100 g of ion-exchanged water was used, and it took 3 minutes. Filter and wash.

(2) Charge and discharge test

In the same manner as in the first embodiment, the constant current constant voltage was charged to 3.9 V at a temperature of 0.25 C at a temperature of 25 ° C, and then a constant current was performed at a current of 0.1 C, 0.2 C, 0.5 C, 1 C, and 2 C, respectively. The discharge was discharged to a charge and discharge cycle of 2.3 V twice, and the discharge capacity of the second time was measured. The discharge capacities of 0.1 C, 0.2 C, 0.5 C, 1 C, and 2 C were 149.7 mAh/g, 147.3 mAh/g, 145.6 mAh/g, 139.5 mAh/g, and 132.2 mAh/g, respectively. The evaluation results are shown in Table 2.

(Example 4)

In the case of Example 4, a battery for evaluation was produced under the same conditions as in Example 1 except for the washing step.

The contents of the specific washing step and the results of the charge and discharge test are as follows.

(1) Washing

After the autoclave was cooled to room temperature, the produced LiFePO _{4 was} taken out, and a liquid (Li concentration: 0.001 mol/L) in which Li ₃ PO _{4 was} dissolved in a ratio of 0.00386 g with respect to 100 g of ion-exchanged water was used, and it took 3 minutes. Filter and wash.

(2) Charge and discharge test

In the same manner as in Example 1, the constant current constant voltage was charged to 3.9 V at a current of 0.02 C at a temperature of 25 ° C, and then 0.1 C, respectively. The currents of 0.2C, 0.5C, 1C, and 2C were subjected to constant current discharge to a charge and discharge cycle of 2.3 V for 2 times, and the discharge capacity of the second time was measured. The discharge capacities of 0.1C, 0.2C, 0.5C, 1C, and 2C were 146.4 mAh/g, 144.4 mAh/g, 140.8 mAh/g, 133.9 mAh/g, and 124.4 mAh/g, respectively. The evaluation results are shown in Table 2.

(Comparative Example 1)

In Comparative Example 1, a battery for evaluation was produced under the same conditions as in Example 1 except for the washing step.

The contents of the specific washing step and the results of the charge and discharge test are as follows.

(1) Washing

After the autoclave was cooled to room temperature, the produced LiFePO _{4 was} taken out, and it was filtered and washed for 3 minutes using ion-exchanged water.

(2) Charge and discharge test

In the same manner as in Example 1, the constant current constant voltage was charged to 3.9 V at a temperature of 0.25 C at a temperature of 25 ° C, and then again at 0.02 C, 0.05 C, 0.1 C, 0.2 C, 0.5 C, 1 C. The current was subjected to constant current discharge to a charge and discharge cycle of 2.3 V for 2 times, and the discharge capacity of the second time was measured. The discharge capacities of 0.02C, 0.05C, 0.1C, 0.2C, 0.5C, and 1C were 144.7 mAh/g, 140.6 mAh/g, 132.2 mAh/g, 122.3 mAh/g, and 111.8 mAh/g, respectively. The evaluation results are shown in Table 2.

(Comparative Example 2)

In Comparative Example 2, a battery for evaluation was produced under the same conditions as in Example 1 except for the carbon coating step.

The contents of the specific carbon coating step and the results of the charge and discharge test are as follows.

(1) Carbon coating

The dried LiFePO ₄ powder was heated in a mixed gas of 98.8% nitrogen and 1.2% oxygen for 90 minutes from room temperature to 700 ° C, and then heated for 5 hours, after which carbon was coated on the LiFePO ₄ particles by natural cooling. This point is different from that of Embodiment 1.

(2) Charge and discharge test

In the same manner as in Example 1, the constant current constant voltage was charged to 3.9 V at a temperature of 0.25 C at a temperature of 25 ° C, and then again at 0.02 C, 0.05 C, 0.1 C, 0.2 C, 0.5 C, 1 C. The current was subjected to constant current discharge to a charge and discharge cycle of 2.3 V for 2 times, and the discharge capacity of the second time was measured. The discharge capacities of 0.02C, 0.05C, 0.1C, 0.2C, 0.5C, and 1C were 45.2 mAh/g, 43.2 mAh/g, 39.3 mAh/g, 28.0 mAh/g, and 18.2 mAh/g, respectively. The evaluation results are shown in Table 2.

(Comparative Example 3)

In Comparative Example 3, a battery for evaluation was produced under the same conditions as in Example 1 except for the carbon coating step.

The contents of the specific carbon coating step and the results of the charge and discharge test are as follows.

(1) Carbon coating

The dried LiFePO ₄ powder was heated from room temperature to 350 ° C in nitrogen for 90 minutes, and then heated for 5 hours, and then carbon was coated around the LiFePO ₄ particles by natural cooling, which is the same as in Example 1. Different.

(2) Charge and discharge test

In the same manner as in Example 1, the constant current constant voltage was charged to 3.9 V at a temperature of 0.25 C at a temperature of 25 ° C, and then again at 0.02 C, 0.05 C, 0.1 C, 0.2 C, 0.5 C, 1 C. The current was subjected to constant current discharge to a charge and discharge cycle of 2.3 V for 2 times, and the discharge capacity of the second time was measured. The discharge capacities of 0.02C, 0.05C, 0.1C, 0.2C, 0.5C, and 1C were 114.8 mAh/g, 109.3 mAh/g, 98.7 mAh/g, 81.0 mAh/g, and 57.0 mAh/g, respectively. The evaluation results are shown in Table 2.

(Comparative Example 4)

In the case of Comparative Example 4, a battery for evaluation was produced under the same conditions as in Example 1 except for the carbon coating step.

The contents of the specific carbon coating step and the results of the charge and discharge test are as follows.

(1) Carbon coating

The dried LiFePO ₄ powder was heated from room temperature to 950 ° C in nitrogen for 90 minutes, and then heated for 5 hours, and then carbon was coated around the LiFePO ₄ particles by natural cooling, which is the same as in Example 1. Different.

(2) Charge and discharge test

In the same manner as in Example 1, the constant current constant voltage was charged to 3.9 V at a temperature of 0.25 C at a temperature of 25 ° C, and then again at 0.02 C, 0.05 C, 0.1 C, 0.2 C, 0.5 C, 1 C. The current was subjected to constant current discharge to a charge and discharge cycle of 2.3 V for 2 times, and the discharge capacity of the second time was measured. The discharge capacities of 0.02C, 0.05C, 0.1C, 0.2C, 0.5C, and 1C were 127.4 mAh/g, 124.5 mAh/g, 116.4 mAh/g, 105.4 mAh/g, and 88.7 mAh/g, respectively. The evaluation results are shown in Table 2.

(Comparative Example 5)

In Comparative Example 5, a battery for evaluation was produced under the same conditions as in Example 1 except for the carbon coating step.

The contents of the specific carbon coating step and the results of the charge and discharge test are as follows.

(1) Carbon coating

The dried LiFePO ₄ powder was heated from room temperature to 700 ° C in nitrogen for 90 minutes, and then heated for 0.5 hour, and then carbon was coated around the LiFePO ₄ particles by natural cooling, which is the same as in Example 1. Different.

(2) Charge and discharge test

In the same manner as in Example 1, the constant current constant voltage was charged to 3.9 V at a temperature of 0.25 C at a temperature of 25 ° C, and then again at 0.02 C, 0.05 C, 0.1 C, 0.2 C, 0.5 C, 1 C. Current through constant current discharge The charge-discharge cycle to 2.3 V was performed twice, and the discharge capacity of the second cycle was measured. The discharge capacities of 0.02C, 0.05C, 0.1C, 0.2C, 0.5C, and 1C were 135.5 mAh/g, 131.1 mAh/g, 123.2 mAh/g, 113.9 mAh/g, and 96.2 mAh/g, respectively. The evaluation results are shown in Table 2.

From the above results, it was confirmed that the discharge capacities of Examples 1 to 4 were high as compared with the comparative examples, and the difference was large particularly at the high rate side (2C). This is considered to be because the elution effect of the lithium metal phosphate by the lithium ion is suppressed by washing with a lithium ion-containing cleaning liquid after the lithium metal phosphate is washed.

Claims (18)

A method for producing a positive electrode active material for a lithium secondary battery, which is characterized by synthesizing a lithium metal phosphate represented by a composition of LiMPO ₄ (the element M is a transition metal of any one or more of Fe, Mn, Co or Ni) The lithium metal phosphate is washed with a lithium ion-containing cleaning solution, and the solvent of the cleaning liquid is water. The method for producing a positive electrode active material for a lithium secondary battery according to the first aspect of the invention, wherein the solute of the cleaning liquid contains LiClO ₄ , Li ₂ CO ₃ , LiOH, LiPF ₆ , Li ₃ PO ₄ , LiH ₂ PO ₄ And at least one of CH ₃ CO ₂ Li. The method for producing a positive electrode active material for a lithium secondary battery according to the first or second aspect of the invention, wherein the pH of the lithium ion-containing cleaning liquid is 5 or more and 9 or less. The method for producing a positive electrode active material for a lithium secondary battery according to the first or second aspect of the invention, wherein the cleaning is performed by setting a temperature of the lithium ion-containing cleaning liquid to 15 ° C or higher. The method for producing a positive electrode active material for a lithium secondary battery according to claim 1 or 2, wherein the washing time is within 1 hour. The method for producing a positive electrode active material for a lithium secondary battery according to the first or second aspect of the invention, wherein the lithium metal phosphate after the synthesis contains either or both of Nb and V. The method for producing a positive electrode active material for a lithium secondary battery according to the first or second aspect of the invention, wherein the particle size of the lithium metal phosphate after the synthesis is 20 to 200 nm. The method for producing a positive electrode active material for a lithium secondary battery according to the first or second aspect of the invention, wherein the number of Li atoms relative to the number of M atoms in the region from the surface of the lithium metal phosphate after the synthesis to a depth of 2 nm The ratio is 0.7 or more and less than 1.1. The method for producing a positive electrode active material for a lithium secondary battery according to claim 1 or 2, wherein after the washing, the carbon-containing layer is formed on at least a portion of the surface of the lithium metal phosphate. The method for producing a positive electrode active material for a lithium secondary battery according to claim 9, wherein the carbonaceous layer is formed on at least a portion of a surface of the lithium metal phosphate after the washing, and the washing is performed. Thereafter, the lithium metal phosphate is mixed with a liquid in which a substance containing a carbon atom is dissolved, and after the mixing, a solvent of the liquid in which the substance containing a carbon atom is dissolved is removed, and the liquid in which the substance containing a carbon atom is dissolved is removed. After the solvent, the mixture of the lithium metal phosphate and the carbon atom-containing material is heated at a temperature of 400 ° C or higher and 900 ° C or lower for 1 hour or longer and 20 hours or shorter in an atmosphere having an oxygen concentration of 1% or less. The method for producing a positive electrode active material for a lithium secondary battery according to the first or second aspect of the invention, wherein the transition metal (M) of the LiMPO ₄ is either Fe or Mn. The method for producing a positive electrode active material for a lithium secondary battery according to the first or second aspect of the invention, wherein the transition metal (M) of the LiMPO ₄ contains both Fe and Mn. The method for producing a positive electrode active material for a lithium secondary battery according to claim 1 or 2, wherein the synthesis is carried out by hydrothermal synthesis. The method for producing a positive electrode active material for a lithium secondary battery according to claim 13, wherein the hydrothermal synthesis system comprises a water-containing liquid, a lithium (Li) source, one or more transition metal (M) sources, and The phosphoric acid (PO ₄ ) source is used as a raw material. The method for producing a positive electrode active material for a lithium secondary battery according to claim 13, wherein the hydrothermal synthesis uses a liquid containing water, Li ₃ PO _{4 ,} and one or more kinds of transition metal (M) sources as raw materials. get on. The method for producing a positive electrode active material for a lithium secondary battery according to claim 14, wherein the transition metal (M) source is FeSO ₄ and/or MnSO ₄ . A method for producing a positive electrode active material for a lithium secondary battery according to claim 14, wherein LiOH is used as the lithium (Li) source. A method for producing a positive electrode active material for a lithium secondary battery according to claim 14, wherein H ₃ PO _{4 is} used as the source of the phosphoric acid (PO ₄ ).